Spring is finally here, and I'm finally back in the UK too so time to give everyone an update - sorry for the delay!!!
The Stage 1 turbos were finished many months ago, and had accumulated over 4000 miles of testing. The testing went without a hitch from the turbos - the only issues I had were not turbo related, but more to do with a poor design of oil return pipe by BMW
More on that later...
The net result of the Stage 1 upgrade, which was compressor wheel + reprofiled exhaust turbine, was an approximate 65hp increase at the flywheel over stock. Torque figures went up by about 50lbft. This was over and above the performance I was already getting from the various engine mods that have been done to my car
The real-world improvement was signifcant - mid-range pull was stronger, making 4th gear feel like 3rd, and there was a noticeable improvement in top-end power with much less tapering off as the revs climbed towards red-line. The car felt as strong at 5000rpm as it did at 3500rpm, whereas a normally-configured engine would start tapering off at 5000rpm, becoming a bit breathless in comparison at 6000rpm.
Low speed pickup was not really hindered - there seemed to be no appreciable increase in turbo lag. Spool up times seemed very quick - from 1300rpm there was considerable shove, and by 1800rpm the car was pulling very hard. I could wheelspin easily in 3rd gear just by being aggressive with the throttle
Fuel economy whilst cruising is totally unaffected, although the consumption increased noticeably when pushing on as the fuel rate is increased to match the rise in air flow. I averaged 28mpg (UK) over the 4000 miles, which is pretty damn good considering I spent two days at Brands Hatch doing my best to break the turbos!!
Roll forward to the end of February, and I sent the car away again to be taken apart so that the various components could be inspected. The turbos came off and were sent back for examination, and for the next stage of the upgrades to take place. It was during this dismantling process of the engine that we noticed the OEM oil return lines have a serious flaw in their design. These lines serve to return the oil from the turbo to the sump. However the line itself, which is made up of an initial fixed metal pipe joining onto a braided line, has an almost 130 degree kink in it's layout. This curve only serves to impede the oil return flow, and therefore gives an increased likelihood to oil pooling in the turbo itself (which I suffered from in the form of a little white smoke coming from the exhausts).
We have therefore decided, in conjunction with the Stage 2 turbo upgrade, to remanufacture the oil lines with extended versions, which will serve to alleviate the existing kink, and will also allow an easier installation of the Stage 2 upgrade on the turbos...
So, to the Stage 2 upgrade...
Originally, the turbo upgrades were going to be done in one go - compressor wheel and exhaust turbine together. However TD wanted to spend more time looking at the exhaust turbine side of things, so we decided to pursue with the Stage 1 upgrades first (all documented previously in this thread) and tackle the Stage 2 job later on.
For those who have yet to catch on, the Stage 1 upgrade is an enlarged compressor wheel with a reprofiled exhaust turbine wheel. Stage 2 further enlarges the compressor wheel over and above Stage 1, and also has an enlarged exhaust turbine wheel.
The Stage 2 process is hugely involved, and presents a serious engineering challenge. Of primary concern was the desire to keep both oil and water cooling for the upgraded exhaust turbine assembly. Using a Garrett core (as others have done) was deemed an unacceptable solution, especially given the heat-induced wear these turbos potentially suffer from. The other main issue was one of physical dimensions - how to fit an enlarged turbine in a small housing, and maintain good airflow.
These were the challenges facing the design team, but after a lot of consideration to wall strength and potential flow constrictions etc, the TD engineers came up with a very intelligent solution. Not only does the upgraded turbine use both water and oil cooling (with the new oil lines manufactured for this application), the compressor wheel is being enlarged again (over and above Stage 1) to maintain the correct flow characteristics.
The Stage 2 turbos are 95% completed now, and I should have them back on Tuesday or Wednesday this coming week. Before the process to Stage 2 began, the turbos were inspected and declared to be in excellent health, with barely any wear or carbonisation after the 4000 miles of Stage 1 testing. This is excellent news, and points to the long-term reliability of these upgraded units. The wastegates and actuator bearing also showed no signs of deterioration
Plans are afoot to pursue a different course in mapping solution for the new engine requirements... more news on that next week!
Pictures below will show the state of the Stage 1 turbos coming off the car, before the processing into Stage 2 variants.
Old turbine housing (Stage 1, 4000 miles)
Old turbine housing & manifold, before cleaning (Stage 1, 4000 miles)
Bearing housing & ringlang, before cleaning (Stage 1, 4000 miles)
Bearing housing with thrust bearing - very little sign of wear (Stage 1, 4000 miles)
Compressor cover prior to Stage 2 conversion
Old core assembly (Stage 1, 4000 miles)
Old core assembly next to new shaft and compressor wheel
Old core assembly next to new shaft
